1. Field of the Invention
The present invention relates to a fluorescent image-printed article for security measures, where the image is formed by making use of an ink containing a fluorescent substance which is not visible under visible light but can be made visible under the irradiation of ultraviolet rays through the emission of fluorescent light in the visible region. The present invention also relates to such a fluorescent ink.
2. Description of the Related Art
In recent years, in order to prevent the forgery or duplication of securities, such as notes and prepaid cards so as to enhance the security of using these securities, there has been practiced the printing of microletters or copy-restraining patterns on these securities by making use of an infra-red absorbing ink or a fluorescent ink. Of these inks, the fluorescent ink can be used for printing so-called hidden marks which are hardly visible under the ordinary visible light but can be made detectable through visual observation due to the emission of fluorescence under the irradiation of ultraviolet rays or infra-red rays.
In the production of the fluorescent ink, fluorescent pigments are employed in place of organic or inorganic color pigments exhibiting absorption in a visible light region and being used in the ordinary printing inks. The fluorescent ink can be employed in the formation of images on a substrate by means of offset printing or thermal transfer printing in the same manner as in the case of ordinary printing inks.
In order to identify fluorescent images formed on an image-printed article, it is required to irradiate the image-printed article with ultraviolet rays employed as an exciting light. When the image-printed article is irradiated with ultraviolet rays, the ultraviolet rays are absorbed by the fluorescent substance of fluorescent images, thereby causing the emission of fluorescence in the visible region. This fluorescence can be recognized through visual observation or using a camera. As for the wavelength of ultraviolet rays to be irradiated, it can be suitably determined depending on the kinds of fluorescent substance to be employed. Therefore, various kinds of light source can be suitably selected according to the kinds of fluorescent substance.
As for specific examples of such a fluorescent ink, it is proposed, in recent years, to employ rare earth metal complex (see for example JP Laid-open Patent Publication (Kokai) No. 2003-26969 and No. 2002-173622). Fluorescent ink employing a rare earth metal complex is advantageous as compared with the ink employing the conventional inorganic fluorescent substance in the respect that it is possible to suppress light scattering at an image portion formed of the fluorescent ink, thereby making it possible to overcome, to some extent, the problem that the presence of the fluorescent ink can be recognized as an image-printed article is observed under visible light. Further, the fluorescent ink employing the rare earth metal complex is also advantageous in that it is possible to suppress the association of ink in the binder of ink more effectively as compared with the fluorescent ink employing an organic fluorescent substance, so that the fluctuation of luminosity due to working temperature can be more effectively suppressed.
The europium complex described in JP Laid-open Patent Publication (Kokai) No. 2003-26969 is constructed such that only β-diketone ligand is coordinated to europium and the ink composition containing this europium complex is capable of emitting red fluorescence. However, this ink composition is low in luminosity, so that it is now desired to further enhance the luminosity thereof.
Further, the europium complex described in JP Laid-open Patent Publication (Kokai) No. 2002-173622 is constructed such that both β-diketone ligand and phenanthroline ligand are coordinated to europium and the ink composition containing this europium complex is also capable of emitting red fluorescence. However, this europium complex is accompanied with a problem that it is prone to crystallize in the binder of the ink and, due to this crystallization, light scattering is caused to generate at an image portion formed from the europium complex. As a result, the presence of the fluorescent ink is made recognizable as an image-printed article having such an image portion is as seen under visible light, thus failing to sufficiently solve the problem of deterioration of security level.